The invention is directed to lead activated luminescent materials and a process for producing them.
Luminescent materials are solid substances which are capable of emitting light after absorbing energy (e.g. daylight, ultraviolet light, rontgen rays (X-rays) or corpuscular rays), which emitted light generally exhibits a greater wavelength than that of the absorbed ray. The light radiation occurs during the excitation itself and at various lengths of time thereafter, which can be from milliseconds to hours.
Luminescent materials find expanding use, such as for example, in lighting engineering to increase the energy yield in mercury lamps, in X-ray diagnostics, or in the coating of television tubes to make visible the modulated electron beam. Besides they are used as coatings for ceramic products, especially in glazes for ceramic tiles.
Only a few inorganic compounds are capable of luminescence. Generally, the luminescence capability is first acquired through an activation, for example, by building small amounts of ions (activators) foreign to the crystal lattice into the crystal lattice (host lattice) of the inorganic compound (base material). Therefore, in the activator containing luminescent materials, it is a matter of crystallized compounds in which a few cations of the base materials are replaced by foreign cations. The built-in cations with the surrounding band of anions thereby has been conceived as the so-called luminescent centers which are responsible for the characteristic absorptions and emissions.
As base materials for the luminescent materials of significance above all are salts of the alkaline earths and their side group elements zinc and cadmium such as phosphates, silicates, borates, aluminates and oxides. As activators there are preferably used the rare earths and heavy metals, such as manganese, lead, tin, or antimony.
The luminescent materials generally must be very pure and have a crystal lattice as free as possible from disturbances since impurities in the starting materials act as quench centers and reduce the luminescent intensity of the luminescent materials. Lattice defects act similarly. The luminescent capability is damaged in the mechanical comminution of the compounds and in the extreme case can be completely destroyed.
The previously known luminescent materials have the further disadvantage that most of them are not temperature stable and disintegrate in many fritted glasses. Therefore, they are not suitable as pigments in glazes for ceramic tiles which require firing temperatures of over 700.degree. to 800.degree. C.
There are only a few luminescent materials which emit in the blue-green region (nm 480 nm), those luminescent materials which have a long afterglow time in this region.
Luminescent materials activated with lead are relatively rare.
In essence, there are known only lead containing barium sulfate and lead containing alkaline earth silicates such as barium strontium magnesium silicate, barium disilicate or barium zinc silicate which have emission bands in the regions of 300 to 370 nm. As with all previously known luminescent materials, they must be produced from highly pure starting materials by solid body reactions at 1150.degree. to 1200.degree. C. Besides the products are sensitive to grinding and not stable against many ceramic glazes.
Therefore, it was the problem of the present invention to develop lead activated luminescent materials which emit in the blue-green region, are excitable with both long wave (366 nm) and also with a short wave (254 nm) ultraviolet light, have afterglow times in the seconds to minutes region, can be produced from materials which are not highly purified and are resistant to customary ceramic glazes. Besides there should be found a process for the production of these luminescent materials.